Various known methods are used in the measurement of infrared radiation. An essential part of the sensor principle is the use of passive infrared detectors (PIR Detectors). This is characterized by its simple and cost-effective production.
These PIR detectors are two-terminal devices, and could be represented in the equivalent circuit diagram by a current source that will deliver a current IPIR dependent on changes in radiation and temperature in parallel with a capacitor CPIR. (See FIG. 2.)
Various problems arise when analysing the signals received by the PIR sensor:
First, the PIR detector's operating points drift as they start to self-charge. Secondly, the current source IPIR, generally, only delivers a very low current at a relatively high internal resistance. This internal resistance RPIR is shown in FIG. 2. These conditions result in the requirement of a wide dynamic range and in a very high internal resistance for the subsequent amplifier and analog-to-digital converter circuit (measurement circuit). In the examples disclosed herein, the analog-to-digital converter circuit is realized with a ΔΣ-converter (ΔΣ).
Due to the high internal resistance of an ideally operating measurement circuit, charges once generated cannot, however, be discharged. This could cause the circuit to exit the measurement circuit's operating point, as it would be overdriven.